Corrugated fiberboard

ABSTRACT

Corrugated fiberboard which resists deterioration in strength when in the presence of moisture or water. Such board is made by treating medium and, optionally, liner members with an aminoplast containing phenol-aldehyde resole resin composition and thereafter bonding corrugated medium to liner members with a thermoplastic waterproof adhesive system containing substantially fully hydrolyzed polyvinyl alcohol.

United States Patent 3,062,699 11/1962 Raphael etal.

Robert P. Carlson Springfield, Mass. Appl. No. 855,666

Filed Sept. 5, 1969 inventor Patented Nov. 2, 1971 Assignees Alton BoxBoard Company Alton, lll. Monsanto Company St. Louis, Mo.

CORRUGATED FIBERBOARD 7 Claims, 2 Drawing Figs.

U.S. Cl 161/133, 117/155 L, 161/264, 260/29.3, 260/5l.5, 260/839,260/840 Int. Cl 1322b 3/28, B32b 27/42, C08g 37/18 Field otSearch117/155 L; 161/133, 264; 260/29.3, 51.5, 840, 839

References Cited UNITED STATES PATENTS Primary ExaminerRobert F. BurnettAssistant Examiner-Stephen M. Hoffman AHomeys-.Iohn W. Klooster, ArthurE. Hoffman, Richard W.

Sternberg and Neal E. Willis ABSTRACT: Corrugated fiberboard whichresists deterioration in strength when in the presence of moisture orwater. Such board is made by treating medium and, optionally, linermembers with an aminoplast containing phenol-aldehyde resole resincomposition and thereafter bonding corrugated medium to liner memberswith a thermoplastic waterproof adhesive system containing substantiallyfully hydrolyzed polyvinyl alcohol.

PATENIEnunve I97I 36175428 IO II 1 1 j TREAT LINER WITH TREAT MEDIUMTREAT LINER WITH RESIN (OPTIONAL) WITH RESIN RESIN (OPTIONAL) L I v vDRY TO PRESET DRY TO PRESET DRY TO PRESET VOLATILES CONTENT VOLATILESCONTENT VOLATILES CONTENT II I CORRUGATE MEDIUM AND APPLY ADHESIVE TOFLUTES THEREOF ASSEMBLE FIBERBOARD 8 HOT PRESS TO SET ADHESIVE AND(OPTIONALLY) RESIN MAKE CARTON BLANKS (SLOT, SCORE, CUT, ETC

HEAT TREAT BLANKS TO CURE RESIN (OPTIONAL) HUMIDIFY BLANKS F|G 2(OPTIONAL) ASSEMBLE BLANKS INVENTOR INTO CARTONS ROBERT P. CARLSONATTORNEY CORRUGATED FIBERBOARD IMPROVED CORRUGATED FIBERBOARD BACKGROUNDCorrugated fiberboard has been used for many years and for variouspurposes, but is well known to have poor wet strength properties.However, for some time, manufacturers of corrugated fiberboard havetried to remedy such inherent deficiencies in wet strength (asdetermined, for example, using postmoisture exposure crush resistancecomparative measurements) by impregnating the paper sheeting used inmaking such board with various polymeric substances including phenolicresins and then using a waterproof thermosetting starch/fonnaldehydebased adhesive to bond together the resin-treated medium and linermembers. So far as is known, however, such efforts have generally notbeen successful in producing corrugated fiberboard of commerciallyuseful quality; see, for example, Koning, Jr. and Fahey of the US.Forest Products Laboratory reported in Package Engineering, Vol. No. 10,Oct. I965 at pages 130 through 139.

Such previous efforts to improve wet strength using phenolaldehyde papertreating resins have generally not been successful because of problems,such as:

A. the prolonged and excessively high temperatures required forphenol-aldehyde resin curing (preventing the use in effect of phenolicresins on the high-speed machinery conventionally commercially used tomake corrugated fiberboard);

B. the inability of conventional aqueous phenol-aldehyde resin systemsto uniformly penetrate base paper stock during high-speed treating;

C. the embrittlement commonly associated with paper stock followingphenol-aldehyde resin treatment and incorporation into corrugatedfiberboard; and the like.

D. the lack of compatibility and good bonding between phenol-aldehydethermoset-resin treated base stock and adhesive systems used to bondsuch resin treated medium and liner sheets together into corrugatedfiberboard; and the like.

In addition, such previous efforts using such treating resins andthermosetting formaldehyde-based adhesives have not been commerciallypracticable because of such problems as the amount of heat required toincrease the adhesive value of a starch/formaldehyde adhesive system toan acceptable level for boxboard manufacture sometimes causes apremature thermosetting of the resin in the resin treated medium or evenliner members.

it has surprisingly and unexpectedly now been discovered that such priorart problems can be overcome when an aminoplast containingphenol-aldehyde paper-treating resin system and a particular class ofpolyvinyl alcohol adhesives are used in conventional high-speedcorrugated fiberboard manufacturing techniques to make corrugatedfiberboard. The product board has among other desirable propertiesgenerally superior post-moisture exposure crush resistance.

A significant advantage associated with the present invention is thatthe resin system used for impregnation of the base paper stock used insuch fiberboard manufacture requires significantly higher temperaturesto thermoset than those needed to set the adhesive system employed,thereby making it possible if desired to maximize adhesive bond strengthin a product carton blank by first setting the adhesive used to make theboard, then forming the blank from the board, and finally curing theresin in the paper used to make the board rather than curing adhesiveand resin at the same time. Consequently, one may first manufacturecorrugated fiberboard having good dry strength and flexibilitycharacteristics, such as are desirable in further machine processing ofcorrugated fiberboard into manufactured articles like box blanks, etc.,and then process such fiberboard into such manufactured articles.Thereafter, the impregnated resin can be thermoset, and the resultingcorrugated fiberboard accordingly stiffened and rigidified, by raisingthe so-manufactured articles to temperatures high enough to thermosetsuch impregnated resin.

The minimal heat input requirements associated with making the productsof this invention are particularly valuable when one is producing boardsuch as double or triple wall corrugated board, or board having heavyliner members, where poor heat transfer characteristics of the productboard require the use of slow machine speeds to effectuate thermosettingof adhesive. Thus, if low heat is required for setting of adhesive, highmachine operating speeds can be employed. One consequence of thisreduced heat requirement is that lower liner preheat temperatures may beused in board manufacture. This means, among other things, that a linercan be preheated with lower temperature-softening thermoplastic coatingpolymers, such as waxes or polyolefins, and still be used to make boardat high speed with minimum machine operating problems.

A particularly significant advantage of the present invention is thatone can use an adhesive which has "quick tack" or green strength suchthat the adhesive can be employed with corrugating machines operating atmaximum speeds with minimum heat applied to achieve bonding betweenmedium and liner members and still produce a rigid-when-wet corrugatedcontainer board.

The thermoplastic adhesive compositions employed in the presentinvention enable one to produce product corrugated board which is notonly rigid when wet but which has a degree of flexibility in the glueline which makes the product board tough without being brittle.

Other and further advantages will be apparent to those skilled in theart from the present teachings.

SUMMARY The present invention relates to a novel corrugated fiberboardconstruction adapted to have good wet strength and crush resistance andto certain novel resin-treated medium and liner sheet members useful asintermediates for the manufacture of such fiberboard. For presentpurposes, wet strength and crush resistance of corrugated fiberboard maybe conveniently measured comparatively using flat crush resistance datameasured on corrugated fiberboard before and after moisture exposure bymeans of ASTM Test Procedure No. D-l225-54.

Sheet members adapted for use in this invention as medium in themanufacture of the corrugated fiberboard utilize paper ranging inthickness from about 7 to 15 thousandths of an inch and having basisweight of from about 25 to 36 pounds per l,000 sq. ft. Such a paper hasbeen treated with from about 5 to 15 weight percent (dry weight basis)of a phenol-aldehyde resin system containing an aminoplast. The paperafter such resin system treatment has a total volatiles content of fromabout 3 to 10 weight percent.

The terms treated," treating, or treatment" as used herein are genericto both impregnation and coating of the substrate paper itself; such canbe accomplished in any convenient, conventional manner. as hereinafterfurther detailed and illustrated.

Sheet members adapted for use in this invention as liners in themanufacture of the corrugated fiberboard utilize paper ranging inthickness from about 5 to 30 thousandth of an inch and having basisweights of from about 25 to pounds per L000 sq. ft. Such a paper hasbeen treated with from about 2 to 10 weight percent (dry weight basis)with such an aminoplast-containing phenol-aldehyde resin system, andsuch paper after such treatment has a total volatiles content of fromabout 3 to 10 weight percent. Liner sheet members used in the corrugatedfiberboard of this invention need not be, but preferably are, so treatedwith such an aminoplast-containing phenol-aldehyde resin system.

A corrugated fiberboard construction of this invention employs a treatedmedium, as above described, and a liner (preferably, though notnecessarily, a treated one, as above described). Each such medium iscorrugated and positioned adjacent to a liner member on one side thereof(preferably there is a liner in each side of a corrugated medium). Eachliner is bonded to its adjoining medium at positions of mutual contacttherebetween with a polyvinyl alcohol based adhesive system which isinterposed between such a liner sheet member and such a medium sheetmember at such positions of mutual contact (typically the tips of thecorrugated medium flutes).

The aminoplast-containing phenol-aldehyde resin system used in thisinvention comprises:

A. from about 1 to 5 weight percent (total dry system basis) ofanammonium salt ofa mineral acid, and

B. the balance up to weight percent (same basis) of anaminoplast-containing resin,

C. said aminoplast-containing resin being selected from the groupconsisting of:

1. condensation reaction products of phenol, formaldehyde, and at leastone aminoplast selected from the group consisting of urea, melamine, anddicyandiamide, and

2. compositions comprising a phenol-formaldehyde resole resin and atleast one such aminoplast, and

D. said aminoplast-containing resin being further characterized byhaving:

I. a mol ratio of combined formaldehyde to phenol ranging from about 1.0to 3.0,

2. a total nitrogen content ranging from about 3 to 20 weight percent(based on total dry resin weight), and

3. a water solubility such that at least about a 30 weight percentaqueous solution of resin solids can be prepared.

Aminoplast containing phenol-aldehyde resins as described above are wellknown to the prior art. Condensation reaction products ofphenol-formaldehyde and at least one aminoplast may be made, forexample, by techniques taught by Mestdagh et al. in US. Pat. No.3,004,941. Aminoplast modified resins of phenol and formaldehyde arepreferred.

Thus, such a modified phenol-formaldehyde resin useable in thisinvention can be prepared by first condensing from about 1.5 to 5 molsof formaldehyde per mol of phenol. Preferentially, the condensation isaccomplished using an alkaline catalyst. The condensation is continueduntil a predetermined free formaldehyde content is reached, asdetermined, for example, by the hydroxylamine hydrochloride test. Asuitable free-formaldehyde content is about 9-15 weight percent based ontotal weight of starting reactants. The formaldehyde used in thestarting reactants can be in the ratio range of about 1.5 to 5 mols permol of phenol, and, preferably, ranges from about 1.5 to 3.5 mols offormaldehyde per mol of phenol. Such aqueous condensation product of.phenol and formaldehyde, having the excess formaldehyde is cooled toabout 30 to 50 C. The aminoplast (dicyandiamide, melamine, or urea, ormixture thereof) is then added in such a proportion that theratio isgenerally, and, preferably, about 1 mol of the aminoplast to about 0.5to 2.0 mols of formaldehyde in the resulting formaldehyde condensationproduct with dicyandiamide, melamine, and/or urea, and more preferably,about 1.2 to 1.6 mols of formaldehyde. For example, when employing amixture of dicyandiamide, melamine, and urea, the mixture can consist offrom about 10 to 90 weight percent of dicyandiamide, and,correspondingly, about 90 to 10 weight percent of urea.

Alternatively, the process for preparing an aminoplast modifiedphenol-formaldehyde resin for use in this invention can be accomplishedby reacting dicyandiamide, melamine, or urea, or mixture thereof, withformaldehyde in the presence of an alkaline catalyzed reaction productof phenol-formaldehyde having no excess free formaldehyde. This processcan be initiated by first reacting phenol with formaldehyde underalkaline catalyzed conditions to provide a water-dilutable condensate ofphenol-formaldehyde having no free formaldehyde. The ratio offormaldehyde to phenol, and of aminoplast to formaldehyde remains asabove indicated.

Other methods known to the art can be used for preparing such a modifiedresin for use in this invention. Typically, such methods involve theseparate preparation of a phenol-aldehyde condensate resin compositionwhich is initially not only water soluble but, also water dilutable tothe extent desired. The dicyandiamide, melamine and/or urea formaldehydecondensation product, as those skilled in the art readily appreciate,can be prepared separately by conventional techniques in the from of aresin which is typically not only water soluble, but also waterdilutable to the extent desired. Such a separately prepared formaldehydecondensation product with dicyandiamide, melamine or urea can have 2 molratio of dicyandiamide melamine and/or urea to aldehyde of from about0.5 to 5. The resin is then added to the preformed phenol-aldehyderesin. Preferably, aminoplast modified phenol-aldehyde resins for use inthis invention have a total combined nitrogen content ranging from about3 to 12 weight percent (dry weight basis), and, in general, thisnitrogen content is less than about 20 weight percent.

A suitable phenol-aldehyde resole resin system can be made by anyconventional procedure known to the art of phenolic resins. For example,one convenient and preferred procedure involves condensing usually attemperatures ranging from about 50 to C. phenol and formaldehyde in theabove-indicated desired mol ratio under aqueous liquid phase conditionsin the presence of a basic catalyst, such as an alkali metal hydroxidesuch as sodium hydroxide or an alkaline earth hydroxide such as calciumhydroxide, a trialkyl amine such as triethylamine, and the like until adesired end point is reached,

such as, for example, a free formaldehyde content which is less thanabout 3 to 10 weight percent.

The product is a phenol-formaldehyde resole resin in aqueous solutionhaving a total solids content of from about 30 to 70 weight percent.Usually and preferably, the resin solution is prepared as a concentrateof from about 40 to 55 weight percent solids (based on the totalsolution weight) which may be conveniently and preferably diluted downbefore use to a solids content of from about 5-50 weight percent.Preferably, the solids content of the concentrate ranges from about45-60 weight percent and preferably the solids content of the dilutedworking solution ranges from about 15-45 weight percent.

For in in the products of this invention, it has been found that thisresole resin should preferably not be advanced in manufacture beyond thepoint where it has a water solubility such that about a 55 weightpercent solids in aqueous solution thereof can be prepared (preferablyabout 30 weight percent). Also, it has been found that this resole resinshould have a methylol content per aromatic ring of from about 0.5 to 3(preferably from about 1 to 2.5) as determined, for example, by NMRmeasurements. If such resin is more advanced, (Le. has a high molecularweight) than such a solubility as above indicated, or if such resin hasa different methylol content than that above indicated, then it appearsto have undesirable paper-treating characteristics, especially at thehigh application speeds conventionally employed for paper transport inthe manufacture of corrugated fiberboard, for purposes of making theimproved products of this invention. It will be appreciated by thoseskilled in the art that, in general, treatment of paper with resin willusually, as a matter of convenience, be accomplished as a separate,preliminary operation before boxboard manufacture is undertaken.

In addition, it has been found that this resole resin should preferablyhave a pH of from about 5.6 to 9.2 when in the form of an aqueoussolution of about 35 weight percent total resin solids (preferably fromabout 7 to 8.5). Also, it has been found that this resole resin shouldpreferably have an ash content of less than about 2.5 weight percentbased on about a 35 weight percent aqueous solution of total resinsolids (preferably less than about 0.7).

One convenient way in which to measure the ash content for presentpurposes is to take 2 grams of such an aqueous solution (35 percentsolids) and deposit same in a crucible. The crucible is then heated toabout l50C. for about 2 hours to substantially completely cross-link theresin and evaporate free water and thereafter the crucible is exposed toabout 540 C. for about 24 hours. Afterwards, the crucible is cooled toroom temperature and measured to determine an increase in weight overstarting empty weight thereby giving the ash content of the resin.

If such resin has a lower or higher pH, or a higher ash content, thanthose respectively above indicated, then it appears to have undesirableeffects upon product paper treated therewith, especially in theproperties of corrugated fiberboard made therefrom, for purposes ofmaking the improved products of this invention.

In making the modified phenol-aldehyde resin systems for use in thisinvention, it is generally convenient and preferred to add the ammoniumsalt and the aminoplast (which when uncombined is preferably urea) inthe respective amounts above indicated, each in the form of a finelydivided powder, or even (preferably) already dissolved in water, to thediluted or partially diluted phenol-formaldehyde resole resin (justdescribed). Such addition causes this pH of this product system to fallin the range from 0.8 to 6, as above described.

Sometimes in order to make the pH of the product phenolaldehyde resinsystem low, yet within the indicated range, one may, if desired, add toa given resole resin solution, preferably diluted for use and with theammonium salt and urea already dissolved therewith, amounts of a stronginorganic acid, such as hydrochloric, or the like, until the pH islowered to some desired value; however, such an acid addition isgenerally not necessary owing to the presence of the ammonium salt whichitself tends to produce a pH within the ranges indicated.

lt is to be noted that, in a resin composition for use in thisinvention, the chemical composition of such an aminoplastphenol-aldehyde resin can itself vary. For example, although during thereaction of the condensates, the phenol, the dicyandiamide, themelamine, and/or the urea will preferentially react with the aldehyde,it is expected that certain other reaction products will also formduring the condensation reaction. These products would be, for example,a phenol-dicyandiamide-formaldehyde reaction product. When employing amixture of dicyandiamide and urea, aphenol-dicyandiamideurea-formaldehyde reaction product can form as wellas a mixture of phenol-dicyandiamide-formaldehyde,phenol-dicyandiamide-urea-fonnaldehyde, and phenol-urea-formaldehydereaction products. It is understood that these reaction products wouldonly exist in minor amounts with the predominant portion of thecondensation reaction products being phenol-formaldehyde anddicyandiamide-formaldehyde, or mixtures of dicyandiamide-formaldehydeand urea-formaldehyde. In general, the preparation of aminoplastmodified phenol-aldehyde is known to those skilled in the art and doesnot form a part of the present invention. As those skilled in the artwill appreciate, the aminoplast modified phenol-aldehyde resins used inthe present invention are of the resole type since not only is thephenol-aldehyde condensation conducted under basic catalysis conditions,but also the aminoplast modification thereof is conducted under basiccatalysis conditrons.

As indicated above, the aminoplast modified phenol-aldehyde resole resinsystem and the ammonium salt are substantially completely dissolved inwater at the time of paper treatment to make products of this invention.Minor amounts, say up to 5 to 7 weight percent of each such material maynot be truly dissolved (especially in concentrates) but such deviationsare within the contemplation of this invention in the wordsubstantially."

The aminoplast modified resin sued is conveniently and preferablyprepared as a concentrate of from about 40 to 55 weight percent resinsolids (based on total resin solutions weight). This concentrate isconveniently and preferably diluted down before use to a resin solidscontent of from about 5 to 50 weight percent. After the ammonium salt isadded to and dissolved in the working solution, the solids content of aconcentrate can range from about 45 to 60 weight percent typically andpreferably the solids content of a diluted working solution ranges fromabout to 45 weight percent.

For use in the present invention, it has been found that an aminoplastmodified resin as described above should not be advanced in manufacturebeyond a point where it has a water solubility such that about a 55especially weight percent solids content aqueous solution thereof can beprepared (preferably about 30 percent). Preferably, this aminoplastmodified resin has a methylol content per aromatic ring of from about0.5 to 3 (more preferably from about I to 2.5) as detennined, forexample, by NMR measurements. If such resin is ore advanced (i.e. has ahigh molecular weight) than such a solubility as above indicated, or ifsuch resin has a different methylol content than that above indicated,then it appears to have undesirable paper treating characteristics,especially at the high application speeds conventionally employed forpaper transport in the manufacture of corrugated fiberboard, forpurposes of making the improved products of this invention.

As indicated above, any ammonium salt of a mineral acid can be used inthe present invention. Examples include ammonium halides (ammoniumchloride preferred), ammonium nitrate, ammonium sulfate, ammoniumphosphate and the like.

It is generally convenient and preferred to add the ammonium salt in therespective amounts above indicated to the solution of aminoplastmodified resin in the form of a finely divided powder or, morepreferably, already dissolved in water. It is preferred that at the timeof use, the ammonium salt be substantially completely dissolved in theworking solution. Preferably, the ammonium salt is not added to theresin solution until shortly before a medium or liner sheet member is tobe treated therewith.

The polyvinyl alcohol adhesive systems employed in this invention areaqueous at the time of application to paper during corrugated fiberboardmanufacture. characteristically, such a system has a total solidscontent of from about 5 to 75 weight percent (about 5 to 35 weightpercent being more preferred, generally) with the balance up to 100weight percent being water. Such systems are well known to the priorart.

A polyvinyl alcohol adhesive system employed in this invention shouldhave a water resistance sufficient to provide a postwater soakpaper-to-paper fiber tearing bond. Such a water-resistant bond isconveniently measured after a 12 hour (24 hours preferred) roomtemperature water soak between two 42 pound basis weight sheets, eachabout 12 thousandths of an inch thick, both sheets being bonded togetherusing an adhesive application rate equivalent to about 4 pounds adhesivesolids per 1,000 sq. ft. of paper surface, and permitting such adhesiveapplication to such a pair of sheets bonded together therewith to dry inair for about 24 hours before such test commences. The sheets are bondedtogether after wet adhesive application before any appreciable moistureloss of adhesive system water has occurred.

Typically, such an adhesive system contains (dry weight basis) fromabout 10 to weight percent of polyvinyl alcohol, the exact amountemployed in any given system depending upon individual circumstances andpreferences, with the balance up to weight percent being selected fromthe group consisting of supplemental binders, fillers, and adjuvants.The polyvinyl alcohol used characteristically has a molecular weightsuch that a 4 weight percent aqueous solution thereof has a viscosity at20 C. of from about 5 to I25 centipoises with about 40 to 70 centipoisesbeing preferred, about 55 to 65 centipoises being more preferred.Molecular weight affects solubility; thus, for example, by using apolyvinyl alcohol having a viscosity of about 60 c.p.s. one can preparean aqueous solution having about 10 weight percent of polyvinyl alcoholtherein if desired. In general, the polyvinyl alcohol used should besubstantially fully hydrolyzed by which reference is had to the factthat a given polyvinyl alcohol used contains not more than about 5weight percent of residual ester groups (based on total polymer dryweight), and preferably not more than about 2 weight percent.Characteristically, if a given polyvinyl alcohol is not so substantiallyfully hydrolyzed, then one does not prepare corrugated fiberboard havingthe desired wet strength and crush resistance characteristicallyassociated with the products of this invention.

Typically, such an adhesive system can contain in addition to thepolyvinyl alcohol supplemental binder materials, fillers, and adjuvants.In general, polyvinyl alcohol adhesive systems as above characterizedcan have the following compositions (based on 100 weight percent totalfor any given composi tion):

Typical supplemental binders employed in the art include materials suchas starch, dextrin and other synthetic polymeric materials such asphenol-aldehyde condensates, urea-formaldehyde condensates, polyvinylacetate, polyacrylates, etc. Typical fillersemployed in the art includematerials such as clay, asbestos, mica, aluminum silicate pigments, woodfloor, etc. Typical adjuvants employed in the art include:

A. fluidizing agents (such as urea, ammonium thiocyanate, etc.) and B.complexing agents such as those which improve tack or improve waterinsolubility, such as boric acid and other boron compounds; or acidicmetal salts having multivalent cations or the like.

Other adjuvants which can be employed as those skilled in the art willappreciate, include viscosity control aids; dyes and pigments;plasticizers and humeotants; stabilizers; and bactericides, germicides,fungicides, etc., and the like. Typical patents describing thepreparation of polyvinyl alcohol adhesives include US. Pat. No.3,320,200; No. 3,371,004; and No. 3,135,648, and the like.

It is preferred to employ polyvinyl alcohol adhesive systems which havequick tack characteristics. Thus, using the quick tack" measuringprocedure described in column 3, lines 33 through 64 of U.S.' Pat. No.3,37l,004, it is generally preferred to employ polyvinyl alcoholadhesive systems having a quick tack strength of about 2.7 to l0.l gramsper square centimeter. Quick tack is especially important in the singlefactor operation in corrugated board manufacture.

As those skilled in he art will appreciate, in the art of corrugatedfiberboard, it has been conventional to employ as the medium sheetmember semichemical paper. A corrugated medium member is formed byrunning a continuous sheet of medium through corrugating rolls. Themedium, or 9 point" as it is sometimes called, takes on a wavelike shapeas it passes between the corrugating rolls which mesh similar to gearteeth except that they are especially shaped to provide contours deemedbest. by a particular manufacturer for corrugations.

While the corrugating medium may be any of the cellulosic fibrous sheetmaterials conventionally used in the art, it is usually a sheet of about26 pounds per [,000 sq. ft. having a thickness of about 0.009 inch forall grades of combined board, but for purposes of the present invention,may be heavier or lighter for special requirements. Corrugating mediumfor example, is most commonly made from semichemical pulp, but is alsomade from straw, kraft, bogus, or chip (mixed, repulped fibers).

There are four conventional or standard sizes of corrugations asfollows:

A 33-39 0185-0110 B 47-53 0.0970.l05 C 39-45 0139-0145 E -97 0045-00624Approximate, depending on thickness of facings and also the particularcornigating rolls.

As with medium sheet members, any conventional liner sheet member can beused in the manufacture of the corrugated fiberboard of the presentinvention. Generally, the liner sheet members are made from sulfatekraft, but sometimes are made from other pulps.

Kraft for liner sheet members is usually made on a fourdrinier machinealthough some is made on a cylinder machine. Commonly, liner sheetmembers are made to standard weights, such as 26, 33, 38, 42, 47, 69,and 90 pounds per 1,000 sq. with thicknesses for such liner sheetmembers ranging from about 0.008 inch to 0.025 inch. Special linerboardcan be used.

Details on the characteristics of medium sheet members and liner sheetmembers are well known to the corrugated fiberboard manufacturingindustry. See, for example, Uniform Freight Classification Rule 41. Thecorrugation flutes can be combined-using adhesive with facing or linersheet member on one side only, called a single face board; when facingsare on both sides of the corrugated medium sheet member, the product issometimes called single wallboard or double face board. if there are twosheets of flutes with a facing on each side and one in the middle, theproduct is sometimes called double wallboard. If there are three layersof flutcs with two outer liner facings and two inner liner facingsbetween medium layers, the product is sometimes called triple wallboard.

The medium and liner can be treated with a resin system by immersion orany other convenient coating techniques. in a product corrugatedfiberboard, only the medium need be resin treated, but preferably, bothliner and medium sheet members are resin treated. For example, in linertreatment, a preferred method involves surface coating rather thanimmersion impregnation. Suitable coating procedures involve applicationto one surface of a liner with a roller coater, doctor blade, or otherapplication mechanism. Such a procedure is particularly applicable whenonly one side ofa liner is to be treated with a resin system because acoating procedure produces a differential impregnation or coating of theliner. Thus, the resin density is then greater relative to one surfaceof the resulting liner sheet than relative to the other (opposed)surface thereof. In general, it is preferred to uniformly coat mediumsheet members with a resin system.

After treatment, a medium or liner sheet member is dried.

' Drying may, for example, be accomplished by passing such over orthrough a hot zone such that the temperature of the liner and/or themedium preferably does not exceed about 107 C. for more than about 0.1second so as to avoid thermosetting the resin system. in drying, wateris substantially completely removed without appreciable advancement orcuring of the resin impregnated into the liner or medium sheet member.Thus, the percentage of volatiles in a treated liner or medium iscontrolled within the range of about 3 to 10 weight percent (totalweight basis). For example, if the percentage of volatiles is reducedbelow such range, the resin system tends to cross-link and subsequentlyduring corrugated fiberboard production reduced adhesive bonding to sucha resin crosslinked medium or liner sheet tends to result, among otherundesired results. On the other hand, for example, if the percentage ofvolatiles is left appreciably above such range, reduced adhesive bondingcan likewise result. Also, outside of these ranges, a treated medium maybe difficult to corrugate. Next, if not stored interveningly, a treatedmedium and a liner member are combined together. Commercially, aconventional corrugating machine may be used for this operation.

Although an adhesive system as described above is generallydiscontinuously but automatically applied only to the flutes of acorrugated treated medium when using machinery to make corrugatedfiberboard of this invention, typical machine adhesive application ratesrange from about 3 to 12 pounds adhesive solids per 1,000 sq. ft. ofproduct corrugated fiberboard but more or less than this amount can beemployed. Adhesive application rates are not critical and can be widelyvaried without departing from the spirit and scope of this invention.

After adhesive application, corrugated medium sheet member(s) and linersheet member(s) are duly combined together, as in a so-calledcorrugating machine, into board, the resulting fiberboard constructionis subjected to temperatures of from about 120 to about 200 C. for timesof from about 5 sec. to 5 minutes 140" to l80 C. for 5 to 30 seconds,preferred) to substantially set the adhesive. Such a product isconverted to one of this invention by heating such to usually somewhathigher temperatures and for usually somewhat longer times sufficient tosubstantially completely thermoset the aminoplast containingphenol-aldehyde resin system in the treated paper of the board. Suchsetting of adhesive and thermosetting of treating resin can beaccomplished in a single step, or in two steps all as hereinafterfurther described and illustrated. Preferred heating temperatures andtimes substantially completely set the adhesive, but do not do more thanpartially thermoset the treating resin thus forming an intermediatefiberboard product. When the adhesive is set and the treating resin onlypartially or even not at all the product fiberboard is generally morelimber and pliable than when the resin is fully thermoset, which isdesirable when the fiberboard product is to be made into box blanks.Preferably, a corrugated fiberboard construction of this invention hastwo liner sheet members.

Usually, and conventionally, a corrugated fiberboard of this inventionis promptly made into box blanks following manufacture, though it ispossible and convenient to store the corrugated fiberboard before sameis used to make box blanks. Box (or carton) blank manufacture is wellknown to those of ordinary skill in the art and does not form part ofthis invention. Conventional box blanks manufacture includes scoring,slotting, and slitting.

If the corrugated fiberboard used to make the blanks does not have itsresin treated medium and (optionally) liner members thermoset, a finalblank processing step is performed. Thus, in such event, usually beforea blank is formed into a box, but after scoring, slotting, and relatedoperations are completed, a resulting carton blank is heated to atemperature and for a time sufiicient to substantially completelythermoset the phenol-aldehyde resin system impregnated into the mediumand (optionally) liner members of the board. In general, suitabletemperatures for this purpose range from about 160 to 235 C. applied fortimes ranging from about 2 to l0 minutes (with higher temperaturesrequiring shorter cure times). Such a thermosetting increases the waterresistance properties (e.g. crush resistance, as indicated above) of thecorrugated fiberboard and blanks made therewith.

DESCRIPTION OF DRAWINGS The invention is better understood by referenceto the attached drawings wherein:

FIG. I is an enlarged, diagrammatic vertical cross-sectional view, someparts thereof broken away, of one embodiment of a corrugated fiberboardconstruction of this invention, and

FIG. 2 is a simplified flow sheet illustrating the method of makingcorrugated fiberboard in accordance with the teachings of the presentinvention.

Turning to FIG. 1, there is seen a corrugated fiberboard construction ofthe present invention which is designated in its entirety by the numeral10. Construction is seen to incorporate a pair of facing liner sheetmembers 11 and 12. Interposed between liner sheet members II and 12 is acorrugated medium sheet member 13. The liner sheet members I1 and 12 areoptionally treated with a aminoplast modified phenolic resin as taughtherein above, while the medium sheet member 13 is treated with anaminoplast modified phenolic resin as taught hereinabove.

The tips of the flutes in the corrugated medium 13 are bonded to theadjacent faces of respective liner members H and 12 by means of apolyvinyl alcohol based adhesive system 14 as taught herein.Conventionally, the adhesive 14 is applied to the tips of the flutesduring the combining operation of the medium 13 and the liners 11 and12. After assembly, the construction 10 is heated as above described toset the adhesive 14 and produce the corrugated fiberboard I0.

Steps in one embodiment of a process for making a fiberboard 10 areshown in FIG. 2. This block diagram is believed to be largelyself-explanatory particularly in view of the foregoing description so nodetailed explanation thereof is given herein. Observe that FIG. 2 merelyillustrates one mode of practicing the present invention and thatdeviations and variations in accordance with the teachings of thepresent invention are possible without departing from the spirit andscope of this invention.

The machine used to make corrugated fiberboard usually combines into asingle operation the steps of corrugating the medium applying adhesivesto the flutes thereof, and assembling the so-prepared medium with liner.Hot pressing of the so-assembled fiberboard is usually also accomplishedin the same machine. Although the process embodiment of FIG. 2 disclosesinitially curing only the adhesive, it will be appreciated that it isconvenient to practice the invention by curing both the adhesive and theresin for treatment of medium and liner members before exposing aproduct corrugated fiberboard to high-moisture conditions.

Those skilled in the art will appreciate that the type of corrugatedfiberboard shown in FIG. 1 and 2 is known to the trade as double facedcorrugated fiberboard since a medium sheet is combined with a so-calledinner and so-called outer liner.

Single faced corrugated fiberboard is made by using a single corrugatedmedium member and a single liner member; double wall corrugatedfiberboard comprises three liners with two corrugated medium membersalternatively spaced between the liners; and triple wall corrugatedfiberboard comprises seven thicknesses and is made by bonding two singlefaced boards into a double faced board in which there are four linersand three corrugated medium members. All such corrugated fiberboardconstructions are within the contemplation of the present invention.

EMBODIMENTS The following examples are set forth to illustrate moreclearly the principles and practice of this invention to one skilled inthe art and they are not intended to be restrictive but merely to beillustrative of the invention herein contained. All parts are parts byweight unless otherwise indicated.

The following examples illustrate resin systems and adhesive systemssuitable for use in making products of this invention. Each such systemhas characteristics as above indicated as being useful in themanufacture of the products of this invention.

EXAMPLE A An aqueous aminoplast containing phenol-aldehyde resin is madeaccording to the teachings of example II of US. Pat. No. 3,004,941. Theaminoplast is dicyandiamide. In the resin is dissolved 4 weight percentof ammonium chloride (based on total resin solids).

EXAMPLE B An aqueous aminoplast containing phenol-aldehyde resin is madeaccording to the teachings of example ll, part B of US. Pat. No. 3,33 I,885. The aminoplast is melamine. In this resin is dissolved 3 weightpercent of ammonium chloride (based on total resin solids).

EXAMPLE C An aqueous aminoplast containing phenol-aldehyde resin is madeaccording to the teachings of example ll, U.S. Pat. No. 3,444,] 19. Theaminoplast is urea and dicyandiamide. In this resin is dissolved 2weight percent of ammonium chloride and 1 weight percent of ammoniumsulfate (based on total resin solids).

EXAMPLE D An aqueous aminoplast containing formaldehyde resin is made asfollows:

In a reaction kettle is placed 200 parts by of 50 weight percentformalin. To the formalin is added 0.43 parts by weight glacial aceticacid solution. The resulting mixture is neutralized with 33 weightpercent aqueous potassium hydroxide solution, and has 7.0. pH rangingfrom about 6.4 to 7.0. Next, to the mixture is added 100 parts by weightof crystalline urea and the mixture is added 100 parts by weight ofcrystalline urea is heated to atmospheric reflux temperatures for about2 hours. Thereafter, the mixture is cooled to 40 C. and neutralized withabout a 33 weight percent potassium hydroxide solution to produce aproduct mixture having a pH of about 7.0 to 8.0. This product mixture isdehydrated under vacuum to the desired solids level (see below). Theyield of urea-formaldehyde resinous product is about 225 weight percent(based on starting urea). This product urea-formaldehyde resin has thefollowing characteristics:

\ Total solids LOOU-ZAOO cps.

Free-formaldehyde (sodium 3.0-6.0:

sulfite method) pH at 25 C. 7.2-8.2 Refractive index at 25 C.1.4900-L4950 Water dilutability 7.5 to l Total nitrogen content lit-24%To prepare an aminoplast containing resin, I parts of the above resinsare mixed with 100 parts of the phenol-fonnaldehyde resole resinprepared in example F (below).

EXAMPLE E An aqueous aminoplast-containing resin is made as follows:

To 100 parts of phenol in a reaction kettle are added first 180 parts of50 percent formalin and then 4 parts .of sodium hydroxide sufl'lcient tobring the pH to about 8.6. The mixture is refluxed for about 3 hours atabout 65 C., while maintaining a reaction pH of about 8.6. Thereafter,this mixture is cooled to about 40 C. This mixture is then neutralizedto a pH of about 7.0 with aqueous diluted hydrochloric acid. Theresulting mixture is dehydrated under vacuum to the desired solids level(see Table A below). The product yield is about 270 weight percent basedon starting phenol. The product has a water solubility such that a 55weight percent aqueous solution of resole resin solids can be prepared.The resin characteristics are summarized in table A below.

Next, a solution comprising 30 parts by weight of urea, and parts byweight of ammonium chloride both dissolved in 60 parts of water isprepared.

A resin system treating solution is made by diluting 50 parts of theresin with 25 parts of water and then adding thereto 8 parts of themodifier solution. The product resin system comprises (on a 100 dryweight percent basis) about 84 weight percent phenol-formaldehyde resoleresin; about 4 weight percent ammonium salt, and about 12 weight percenturea. The system has a pH of about 1.5 when in the form of an aqueoussolution of about 35 weight percent total resin system solids. As thusmade however, this system contains about 30 weight percent total resinsystem solids with the balance up to 100 weight percent being water.

EXAMPLE F An aqueous aminoplast containing resin is made as follows;

To parts of phenol in a reaction kettle are added first 170 parts of 50percent formalin and the 6 parts of calcium hydroxide sufficient tobring the pH to about 8.6. The mixture is refluxed for about 4 hours at62- C. Thereafter, the mixture is cooled to 40 C. The mixture is thenneutralized to a pH of 7.2 with carbon dioxide. The resin is filtered toremove the calcium carbonate. The product has a water solubility suchthat a 55 weight percent aqueous solution of resole resin solids can beprepared. The resin characteristics are summarized in table A below.

Next, there is prepared a solution comprising 400 parts by weight ofurea, and parts by weight of ammonium chloride in 600 parts be weight ofwater.

A resin system treating solution is made by diluting 100 parts of theresin with l00 parts of water and adding thereto 12 parts of themodifier solution. The product resin system comprises (on a 100 dryweight percent basis) about 20 weight percent phenol-formaldehyde resoleresin; about 1.5 weight percent ammonium salt, and about 3.5 weightpercent urea. The system has a pH of about 2.0 when in the form of anaqueous solution of about 35 weight percent total resin system solids.As thus made, however, this system contains about 25 weight percenttotal resin system solids, with the balance up to I00 weight percentbeing water.

EXAMPLE G An aqueous aminoplast containing resin is made as follows:

To 100 parts phenol in a reaction kettle are added 150 parts of 50percent formalin and 4 parts triethylamine amine as a catalyst. Themixture is refluxed at about 70 C. for about 4 hours until a freeformaldehyde end point of 8.6 percent is reached. The resin product isthen cooled to 30 C. lt has a solids content of about 48.0 weightpercent. The product has a water solubility such that a 55 weightpercent aqueous solution of resole resin solids can be prepared. Theresin characteristics are summarized in table A below.

Next, there is prepared a solution comprising 50 parts by weight of ureaand 30 parts by weight ammonium sulfate in 100 parts by weight water.

A resin system treating solution is made by diluting 100 parts of theresin with l00 parts of water and adding thereto 20 parts of themodifier solution. The product resin system comprises (on a 100 dryweight percent basis) about 20 weight percent phenol-formaldehyde resoleresin; about 1.5 weight percent ammonium salt, and about 4.0 weightpercent urea. The system has a pH of about 2.2 when in the form of anaqueous solution of about 35 weight percent total resin system solids.As thus made, however, this system contains about 25 weight percenttotal resin system solids with the balance up to 100 weight percentbeing water.

chloride method.

2 pH measured at 25 C. using an aqueous solution of about 35 weightpercent total resin solids.

Measured by methylol content and methylene bridges per aromatic ringusing nuclear magnetic resonance on a sample of product.

4 Ash content determined using a 35 weight percent aqueous solution oftotal resins at 150 C. at 2 hours and thereafter pyrolyzed for 24 hoursat 540 C.

EXAMPLE H A polyvinyl alcohol adhesive as described in example 1 of U.S.Pat. No. 3,324,065 is prepared.

EXAMPLE 1 A polyvinyl alcohol adhesive as described in example 1 of US.Pat. No. 3,320,200 is prepared.

EXAMPLE J A polyvinyl alcohol adhesive as described in example 2 of U.S.Pat. No. 3,135,648 is prepared.

EXAMPLE K EXAMPLE 1 Each side of a 26 pound basis weight medium papersheet about 0.009 inch thick are roller coated with aminoplastcontaining phenol-aldehyde resin system of example A to a total resinscontent of about 8 weight percent based on the dry weight of the sheetplus resin. One side of a 42 pound basis weight kraft liner paper sheetabout 0.012 inch thick is roller coated with the same resin system to atotal resins content of 4 percent based on the dry weight of the sheetplus resin. The so-treated medium sheet and liner sheet are each driedto a total volatiles content of about 6 percent (as indicated by dryingthe paper to 160 C. for 10 minutes to determine weight loss).

Next, a medium sheet as so prepared in corrugated into type B flutes ofabout 50 per foot, and the polyvinyl alcohol adhesive of example H isapplied to the tips of he flutes of the medium corrugations at the rateof about 8 pounds adhesive solids per 1,000 ft. of product fiberboard.This medium is then combined with two pieces of such liner sheet asprepared above one on each side of the adhesive treated medium sheet soas to form a corrugated fiberboard. This board is now exposed to atemperature of about 160 C. for about seconds to dry and set theadhesive. This product can be considered to be an intermediatecorrugated fiberboard product which may be converted as herein belowdescribed into a corrugated fiberboard product of this invention.

One portion of this intermediate product board is exposed to about 180C. for about 4 minutes to thermoset the treating resin and, thereby,produce product corrugated fiberboard of this invention. This fiberboardis then converted into a first set of box blanks by cutting, slotting,and scoring.

A second portion offiiis 11151112555 BEEICREI'E is converted into asimilar but second set of box blanks by cutting, slotting, and scoring.These blanks are then exposed to about 180 C. for about 4 minutes tothermoset the treating resin and thereby produce product corrugatedfiberboard of this inventron.

The first and the second box blanks are then formed into containers, andeach such respective container is compression tested for dry and wetstrength (top to bottom) using ASTM Test Procedure No. D-642-47. Acontainer of similar dimensions is made from a blank composed ofconventional untreated medium and liner members and the adhesive ofexample H. The compression tests are conducted on containers which areexposed before testing to 50 percent relative humidity for 24 hours atabout 22 C. and also to a water immersion for 24 hours at about 22- C.The results show that the treated containers retain to 40 percent oftheir original compression strength after water immersion whereas theuntreated containers retain substantially no compression strength afterwater immersion.

Samples of corrugated board from the first box blanks, and samples ofcorrugated board from the second box blanks are tested, along withsamples of corrugated board from the untreated box blanks, for flatcrush resistance before and after water immersion using ASTM TestProcedure D-1225-54. The results show that the treated board retainssubstantial strength after water immersion whereas the untreated boardretains substantially no strength after water immersion.

EXAMPLE 2 Using the procedure employed in example 1 to make the secondblanks of that example, but instead employing as the medium one havingtype C flutes and using a 33 pound basis weight paper 0.011 inch thickhaving a 10 weight percent treating resin content and a 7 percentvolatiles content, and further employing as the liner :1 69 pound basisweight paper 0.020 inch thick having a 5 weight percent treating resincontent and an 8 percent volatiles content, blanks and then containersare made and similarly compression tested (top to bottom). The treatedcontainers display excellent compression strength after water immersion.Samples of the treated corrugated board used in such blanks aresimilarly (as in example 1) evaluated for flat crush resistance and arefound to have excellent flat crush resistance after water immersion.

EXAMPLE 3 Using the procedure employed in example 1 to make the secondblanks of that example, but instead employing as the medium one havingtype C flutes and using a 33 pound basis weight paper 0.009 inch thickimpregnated with 10 weight percent of the resin treating solution ofexample B and dried to a 5 percent volatiles content, and insteademploying and untreated liner paper having a 42 pound basis weight and athickness of 0.012 inch. The product board is heated to 180 C. for 10seconds, and then is cut, scored, and slotted to form a box blank. Theproduct blank is heated in a forced draft oven for 2 minutes at 200 C.to complete resin curing. The product blanks are formed into containersand evaluated for compression strength as in example 1, and board usedin the blanks is evaluated for flat crush resistance as in example 1.The results show the product board and blank to have excellent flatcrush resistance and excellent compression strength, respectively, afterwater immersion.

EXAMPLES 4 through 8 Using the procedure employed in example 1 to makethe first board and blanks there described, a series of corrugated boxblanks are, made, and evaluated as in example 1. In each case, themedium, the liner, the treating resin and the adhesive are as describedin table 11] below. The corrugated board and blanks made therefromdisplay excellent crush resistance after water immersion evaluated bythe procedures described in example 1 above.

TABLE III Treating resin system Amount in Arn0unt i;

medium, liner,

Medium 1 Liner percent percent Amount Ex. Thickness Basis ThicknessBasis Ex. Ex. adhesive No. (in inches) weight (in inches) weight No.Resin Vol. Resin Vol. No. applied .009 26 .020 60 C 12 8 3 5 H 6 .011 33.020 69 D 10 6 7 8 I 6 .011 33 .020 69 E 12 7 J 6 .009 26 .020 69 F 12 83 5 H 6 .009 26 .020 69 G 11 5 5 5 K 6 l Flutes in corrugated medium areType B (about per foot). 2 Lbs. adhesive solids/1000 It. of productboard.

when wet comprising:

A. as a medium, 3 corrugated cellulosic sheet member having a thicknessof from about 7 to 15 thousandths of an inch and a basis weight of about25 to 36 pounds per l,000 sq. ft., said sheet member containing from toweight percent (dry weight basis) of an aminoplast containingphenol-aldehyde resin system said sheet member having a total volatilescontent of from about 3 to 10 weight percent, said aminoplast containingresin being selected from the group consisting of (a) condensationreaction products of phenol, formaldehyde, and at least one aminoplastselected from the group comprising of urea, melamine, and dicyandiamideand (b) compositions comprising a phenol-formaldehyde resole resin andat least one such aminoplast, said aminoplast containing resin beingfurther characterized by having (1) a mol ratio of combined formaldehydeto phenol ranging from about 1.0 to 3.0; (2) a total nitrogen contentranging from about 3 to 20 weight percent (based on total dry resinweight); and (3) a water solubility such that at least about a 30percent aqueous solution of resin solids can be prepared,

B. as a liner, a cellulosic sheet member having a thickness of fromabout 5 to 30 thousandths of an inch and having a basis weight of fromabout to 120 pounds per 1,000 sq. ft.

C. as a bondingcomposition, a polyvinyl alcohol adhesive systemcontaining from about 2.5 to 35 weight percent of substantially fullyhydrolyzed polyvinyl alcohol having a molecular weight such that a 4weight percent aqueous solution thereof has a viscosity at 20 C. of fromabout 5 to 125 centipoises withthe balance up to weight percent (dryweight basis) being selected from the group consisting of supplementalbinders, fillers and adjuvants, said adhesive system having a post watersoak, paper to paper fiber tearing bond,

D. said corrugated medium sheet member being positioned adjacent saidliner sheet member and being bonded thereto at positions of mutualcontact therebetween by said bonding composition, and

E. such resulting construction havingbeen subjected to a temperature offrom about to 200 C. for, times of from about 5 seconds to 5 minutes.

2. A construction of claim 1 which has been subjected to temperaturesand times sufficient to substantially completely thermoset saidaminoplast containing phenol-aldehyde resin system.

3. A box blank prepared from a corrugated fiberboard construction ofclaim 1.

4. A box blank prepared from a corrugated fiberboard construction ofclaim 2.

S. The box blank of claim 3 wherein said blank has been subjected totemperatures and times sufficient to substantially completely thermosetand aminoplast-containing phenol-aldehyde resin system.

6. A construction of claim I wherein said liner member is treated withfrom about 2 to 10 weight percent (dry weight basis) with a saidaminoplast containing resin and has a total volatiles content of fromabout 3 to 10 weight percent.

7. A construction of claim 6 which has been subjected to combination oftemperatures and times sufficient to substantially completely thermosetsaid aminoplast-containing phenol-aldehyde resin system.

2. A construction of claim 1 which has been subjected to temperaturesand times sufficient to substantially completely thermoset saidaminoplast containing phenol-aldehyde resin system.
 3. A box blankprepared from a corrugated fiberboard construction of claim
 1. 4. A boxblank prepared from a corrugated fiberboard construction of claim
 2. 5.The box blank of claim 3 wherein said blank has been subjected totemperatures and times sufficient to substantially completely thermosetand aminoplast-containing phenol-aldehyde resin system.
 6. Aconstruction of claim 1 wherein said liner member is treated with fromabout 2 to 10 weight percent (dry weight basis) with a said aminoplastcontaining resin and has a total volatiles content of from about 3 to 10weight percent.
 7. A construction of claim 6 which has been subjected tocombination of temperatures and times sufficient to substantiallycompletely thermoset said aminoplast-containing phenol-aldehyde resinsystem.